idiosyncrasy (noun) – a physiological or temperamental peculiarity; an individualizing characteristic; eccentricity
BREAKDOWN: IDIO- (peculiar) + SYN- (together) + CRAS- (mixture) + -Y (state of)
— idiosyncratic means peculiar to a specific individual
This word is built on the root IDIO-, which means peculiar, personal, or private.
Common IDIO- words include idiom and idiot.
More esoteric IDIO- words include idiograph, idiolect, and idiopathy.